to ovary when growth is at about half the final body size (which is 5–10 mm in most
species).
Pseudothecosomata
(^) These are pteropods with a gelatinous pseudoconch or false shell. In this group, the
mineral shell has been lost altogether, and the visceral organs are surrounded by the
slipper-shaped pseudoconch, which is internal to a very thin skin. These are also
filter-feeders. Elaborate balloons of mucus set out into the water are used for feeding
and suspension. Body size can be as much as 10 cm. All pseudothecosomes are
subtropical or tropical. The most common genera are Corolla and Gleba (Plate 6.8).
Gymnosomata
(^) These carnivorous, unshelled pteropods are little cones of muscle, typically about 1
cm long with two small wings. Most specialize in eating shelled pteropods. A
common high-latitude genus is Clione, a predator of Limacina. It pulls the soft tissue
from the shell using its extrusible radula, then drops the usually undamaged shell,
which falls to the ocean floor. Other genera (e.g. Paedoclione, Crucibranchaea) have
tentaculate structures around the head and extrusible spines for grabbing prey. Apart
from Clione, very little is known of gymnosome life and times.
A Prosobranch Group
(^) The Prosobranchia are a large group of mostly benthic snails is represented in the
plankton by the Heteropoda. Many of the heteropods retain a calcite shell. In Atlanta,
which are the smallest representatives (to about 5 mm), the shell is coiled all the way
around to the opening, and the animal can withdraw completely into it. In Carinaria,
typical of the larger heteropods, the body has enlarged and no longer fits in the shell,
which takes the form of a conical hat covering the gonad and digestive gland. The rest
of the body is a transparent tube with a large feeding structure anteriorly, an
undulatory tail, and a ventral fin opposite the shell. They have eyes located on the
trunk and directed to look beyond the mouth. Black retinal elements and spherical
lenses imply that these eyes are image forming. Carinaria swim with the ventral fin
and mouth directed upward, presumably taking most prey from below by grasping
them with the radular tooth rows. Carinaria japonica can be as large as 30 cm long.
Variations on this body plan are found in Pterotrachea and Cardiopoda.
Arthropoda
(^) Class Crustacea represents this diverse phylum in the plankton. Crustaceans
contribute by far the largest number of zooplankton species, and they are the largest